1. Field of the Invention
This invention relates to a wide-angle lens for electronic imaging using a solid-state image sensor, such as a CCD, and to a superwide-angle lens which has a long back focus with an angle of view of about 75 degrees.
2. Description of Related Invention
Conventionally, wide-angle lenses are used mainly in silver halide photographing optical systems. One example of such wide-angle lenses is disclosed in Japanese Patent Publication (After Examination) No. 57-54767. However, in recent years, wide-angle lenses have been used in electronic photographing cameras, such as electronic still cameras and ITV cameras, the latter being in great demand as monitor cameras. A wide-angle lens used in a ITV camera is disclosed in, for example, Japanese Patent Publication (After Examination) Nos. H1-53765 and H8-20593.
Since an ITV camera is often used as a monitor camera, the lens used in such an ITV camera is required to have a wide angle of view. In addition, because filters are inserted in the lens system of the ITV camera for the purpose of protecting the camera tube, the back focus must be sufficiently long. Similarly, when a wide-angle lens is applied to an electronic still camera, a sufficiently long back focus is also required because an optical low-pass filter and a color separation prism are inserted in the lens system. Furthermore, in order to avoid shading, an adequate quantity of ambient light must be taken in and, accordingly, the exit pupil must be positioned sufficiently far.
However, all of the wide-angle lenses disclosed in the publications mentioned above have one or more problems, for example, insufficient back focus, insufficient compensation for the lens aberrations, or insufficient distance to the exit pupil. For these reasons, the conventional wide-angle lenses are not suitable to a high-performance electronic imaging wide-angle lens or a superwide-angle lens.